Method and apparatus for absorbing thermal energy

ABSTRACT

The subject invention pertains to a method and apparatus for storing thermal energy. The subject thermal energy storage apparatus can function as a heat absorber in a cooling system. A cooling system can incorporate a cooling cycle that utilizes thermal energy storage and has two coolant loops. The primary cooling loop acquires the waste heat from a heat source, such as an electronic device, by heat transfer to the primary coolant via, for example, a sensible heat process (where sensible heat is heat absorbed or transmitted by a substance during a change in temperature which is not accompanied by a change of state) or by evaporating the primary coolant through a latent heat phase change process. The waste heat absorbed by the primary coolant is transferred to the host material of the heat absorber. The subject invention uses a high thermal conductivity host material to house a lower thermal conductivity phase change material, in order to achieve a thermal energy absorber that has a high effective thermal conductivity. In a specific embodiment, the high thermal conductivity host material has have voids within the structure that can be filled by the phase change material. The increased surface area of phase change material in thermal contact with the host material per volume of phase change material allows the thermal energy to be stored or released quickly, because of the enhanced effective thermal conductivity.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application claims benefit of U.S. Provisional ApplicationSer. No. 60/539,153, filed Jan. 26, 2004, which is hereby incorporatedby reference herein in its entirety, including any figures, tables, ordrawings.

FIELD OF THE INVENTION

The subject invention pertains to a method an apparatus for storingthermal energy. The subject invention also relates to a method andapparatus for absorbing thermal energy from a heat source. In a specificembodiment, the subject thermal energy storage apparatus can function asa heat absorber in a cooling system.

BACKGROUND OF THE INVENTION

Many heat absorption and/or heat storage applications have high heatloads, require precise temperature control, and/or have limited heatrejection options. Such applications include, but are not limited to,cooling microelectronics such as computer processors, cooling powerconditioning equipment for prime power generation, cooling high-powerlaser diodes, cooling solid-state laser systems, cooling high powermicro-wave systems, and cooling electronics in spacecraft applications.

Thermal energy storage devices typically include a housing, whichencases a bulk volume of phase change material. Heat is conductivelytransferred from the heat source through the housing structure andbegins to melt the phase change material, effectively storing the wasteheat. Paraffin's are often selected as the phase change material inthermal energy storage devices because of their wide range of meltingtemperatures (−30° C. to 110° C.) and their high latent heatcharacteristics (150-250 kJ/kg). The disadvantage of most phase changematerials is that these materials typically have a low thermalconductivity (k), especially in the liquid phase. Typical thermalconductivities are less than 0.2 W/m-K. This can result in largetemperature gradients in the device being cooled in order to melt theentire volume of phase change material contained in the thermal energystorage device. In a typical phase change process a melting front isseen during the heat transfer process. The front is a dividing linebetween the melted material and the non-melted material. Heat isconducted across this front in order to melt the solid phase of thephase change material.

Some semiconductor devices, such as laser diodes, require thetemperature control to be within ±2° C. and cannot tolerate largetemperature increases during operation. In fact, with respect to thisproblem, the phase change material thermal conductivity is typicallyabout 2-3 orders of magnitude too low. Due to the low thermalconductivity associated with these types of thermal energy storagedevices these systems are limited in the rate at which they can absorbheat.

BRIEF DESCRIPTION OF INVENTION

The subject invention pertains to a method and apparatus for storingthermal energy. The subject invention also relates to a method andapparatus for absorbing thermal energy from a heat source. The subjectinvention also relates to a method and apparatus for enhancing thethermal performance of a thermal energy storage device that absorbsthermal energy from a heat source. The subject invention uses a highthermal conductivity host material to house a lower thermal conductivityphase change material, in order to achieve a thermal energy absorberthat has a high effective thermal conductivity. In a specificembodiment, the high thermal conductivity host material has have voidswithin the structure that can be filled by the phase change material.The increased surface area of phase change material in thermal contactwith the host material per volume of phase change material allows thethermal energy to be stored or released quickly, because of the enhancedeffective thermal conductivity. The quick storage of thermal energy canenable the subject device to be used to cool electronic devices withvery small temperature increases in the electronic device that is beingcooled.

Advantageously, the high conductivity host material can rapidly transferthe heat to the thermal energy storage system throughout a portion of,or the entirety of, the volume of the phase change material. The highconductivity host material of the subject invention conducts the thermalenergy through the host material and into the phase change material overa large surface area of thermal contact between the host material andthe phase change material. This feature can produce a very high rate ofheat absorption.

A specific embodiment for the subject invention can be incorporated withhigh power diode-pumped solid-state laser systems with laser outputpowers, for example, ≧100 kilowatts. These laser systems fire numerouslaser light bursts over a short period of time (<1 minute) and cangenerate over 900 kilowatts of waste heat throughout the entire laseroperation. The surface to which the laser device is mounted must bemaintained at or below 20° C. to maintain diode operating temperaturesat 60°; requiring the coolant loop operating temperatures to be belowambient (e.g. 5-10° C.). If a cold temperature heat sink operating at atemperature lower than the coolant loop (e.g. ≦0° C. is unavailable, arefrigeration cycle is required to reject the laser waste heat. Toremove over 900 kW using a refrigeration cycle in real time wouldrequire an extremely large refrigeration unit and would not be practicalfor most applications. Incorporating the subject thermal energy storageunit as a heat absorber in the laser cooling loop can allow the laserwaste heat to be absorbed and stored in the subject thermal energy unitduring the laser operation time and slowly rejected over a longer periodof time when the laser is non-operational, greatly reducing therefrigeration cycle volume and mass needed. The subject method can alsobe applied to the thermal management of power electronics and other heatgenerating systems where the duty cycle is low. An example of a low dutycycle is a device that produces a large amount of heat for 1 second to60 seconds, and then does not produce heat for 100 seconds to 600seconds.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows the thermal energy generation for four heat loads, Q=1 kW,Q=50 kW, Q=100 kW and Q=250 kW and laser operation times from 0 to 300seconds.

FIG. 2 shows varying total porosity of a host material (0% to 100%)versus the effective thermal conductivity of the heat absorber, for aphase change material having a thermal conductivity of 0.2 W/m-K and ahost material having a thermal conductivity of 540 W/m-K.

FIG. 3 shows the thermal energy storage capacity (J/cm³) of the heatabsorber matrix for total porosities ranging from 0 to 100% at fourdifferent open porosities (65%, 75%, 85% and 95%), for a phase changematerial having a latent heat of fusion of 200 kJ/kg and a liquiddensity of 800 kg/m³.

FIG. 4 schematically illustrates an experimental set-up for studying theheat transfer properties of graphite foam filled with paraffin.

FIG. 5 shows experimental data from experiments performed with theexperimental set-up shown in FIG. 4.

FIG. 6A schematically illustrates a heat absorber in accordance with thesubject invention.

FIG. 6B shows a closer view of the heat absorber of FIG. 6A, showing thehost material and voids in the host material.

FIG. 7A schematically illustrates a heat absorber in thermal contactwith a heat source in accordance with a specific embodiment of thesubject invention.

FIG. 7B schematically illustrates an isometric view of the heat absorberof FIG. 7A.

FIG. 8 schematically illustrates a closed cycle cooling system that usesa specific embodiment of the subject invention

FIG. 9A schematically illustrates the specific embodiment of the subjectinvention that will function as a heat absorber in the cooling cycledepicted in FIG. 8.

FIG. 9B schematically illustrates a front view of the heat absorber ofFIG. 9A, showing coolant tubing embedded in the heat absorber.

DETAILED DESCRIPTION OF INVENTION

The subject invention pertains to a method and apparatus for storingthermal energy. In a specific embodiment, the subject thermal energystorage apparatus can function as a heat absorber in a cooling system.In a further specific embodiment, a cooling system can incorporate acooling cycle that utilizes thermal energy storage and has two coolantloops. The primary cooling loop acquires the waste heat from a heatsource, such as an electronic device, by heat transfer to the primarycoolant via, for example, a sensible heat process (where sensible heatis heat absorbed or transmitted by a substance during a change intemperature which is not accompanied by a change of state) or byevaporating the primary coolant through a latent heat phase changeprocess.

The waste heat absorbed by the primary coolant is transferred to thehost material of the heat absorber. In a specific embodiment, the wasteheat is transferred to the host material of the heat absorber by tubingthat can have, for example, a circular, square, rectangular, or othercross-sectional shape. The tubing, or plumbing, can be fabricated frommaterials such as, but not limited to, aluminum, stainless steel, andcopper. The plumbing material is preferably compatible with the workingfluid in the primary cooling loop. Heat can, preferably, be transferredthrough the plumbing wall. In a specific embodiment, a manifold can beused to distribute the heated coolant from the primary loop through aseries of plumbed channels that are embedded in the thermal energystorage host material. The heat from the primary coolant loop istransferred through the wall of the plumbing to the high thermalconductivity porous host material, which disperses the heat quicklythroughout the host material and melts the phase change materialabsorbing the heat from the primary coolant through a latent heattransfer process.

In a further specific embodiment, the energy stored in the heat absorberis later removed through a second series of embedded plumbed tubes of asecondary coolant loop that cools, for example freezes, the phase changematerial, effectively dissipating the heat from the heat absorber. Thesecondary coolant loop can be a water chiller, a refrigeration cycle, acool liquid pump loop, or some other cooling technique that removesheat. The plumbing for transferring heat from the heat absorber to thesecondary coolant loop can have, for example, a circular, square,rectangular, or other cross- sectional shape, and can be fabricated frommaterials such as, but not limited to, aluminum, stainless steel, andcopper. In a specific embodiment, the secondary coolant loop can befabricated from similar materials and have similar cross sectionalgeometries as the primary cooling loop.

In a specific embodiment, the heat absorber host material can be aporous matrix of graphite, metal foam, or other porous media. Additionalspecific embodiments of the subject invention can incorporate hostmaterials having single or multi-layer mesh structures, finned or ribbedgeometries, structures with honeycomb characteristics, and/orelectroplated patterns. Such features can enable the host material toform void volumes within the material that can be filled by a phasechange material. The host material can be fabricated from, for example,metals, ceramics, graphite materials, diamonds, and/or plastics thathave relatively high thermal conductivities. The thermal conductivityand porosity of the thermal energy storage host material can be selectedto achieve the results needed by the application being addressed. Phasechange materials melt and solidify within a certain temperature rangeallowing energy to be stored and released at nearly a constanttemperature. Typical phase change materials include, but are not limitedto, water, organic paraffin, oils, and inorganic salts.

The function of the thermal energy storage heat absorber is to rapidlyremove heat from the primary coolant loop, during a specified timeperiod, storing the heat in the phase change material. The stored energyis then removed by the secondary coolant loop over a longer period oftime that can be, for example, at least 2 times, at least 5 times, atleast 10 times, at least 30 times, at least 50 times, and/or at least100 times as long as the absorption period. The amount of thermal energythat must be absorbed by the heat absorber is a function of the heatload, Q, generated by the heat source, and the totalE_(stored)=Q·t_(on)   (1)operation time, t_(on) (Equation 1). FIG. 1 illustrates the relationshipbetween the thermal energy generated and the operation time for severalheat loads (Q=1 kW, Q=50 kW, Q=100 kW, Q=250 kW). The rate that heat isreleased, or removed, from the heat absorber, Q_(released), depends onthe amount of energy stored in the heat absorber and the time durationthat the heat is being removed from the absorber, t_(released) (wheret_(released) can be t_(off), t_(off)+t_(on), or some other period oftime, where t_(off) is the time period that the heat source is notgenerating heat) (Equation 2).

$\begin{matrix}{Q_{released} = \frac{E_{stored}}{t_{released}}} & (2)\end{matrix}$In a specific example, if a heat source is generating 100 kW of wasteheat during a 30 second operating period, and then releases that heatover a 300 second time period, the heat released from the absorberduring the 300 second period would be 10 kW. The heat can be releasedwhile the heat source is not generating waste heat and/or while the heatsource is generating heat, or some portion of one or both of theseperiods of time.

Three important properties of a host material are: the total porosity(ε_(total)), the open porosity (ε_(open)), and the thermal conductivity(k). The total porosity is the total percentage of void volume of openspace in the material whether the space is interconnected to other voidsor closed. In a preferred embodiment, the subject invention utilizes ahost material having a porosity in the range from about 0.2 to about0.8; in a more preferred embodiment in the range from about 0.5 to about0.8; and in an even more preferred embodiment a total porosity in therange from about 0.6 to about 0.75. The open porosity of the materialrefers to the percentage of the volume of the voids that is open orinterconnected and will allow the phase change material to fill thevoid. In a preferred embodiment, the subject invention utilizes a hostmaterial having an open porosity in the range from about 0.6 to about1.0; in a more preferred embodiment in the range from about 0.8 to about1.0; and in an even more preferred embodiment an open porosity in therange from about 0.95 to about 1.0.

Another consideration when selecting the host material for the subjectinvention is the average pore size and its impact on spatial poredistribution within the host material. The individual pore shape can befor example, circular, square, hexagonal or some other irregular shape.A preferred embodiment of the subject invention incorporates a hostmaterial having an average pore size in the range from about 50 to about1000 microns; in a more preferred embodiment in the range from about 100to about 500 microns; and in an even more preferred embodiment in therange from about 300 to about 400 microns. Preferably, there should befew, if any, individual pore diameters, so long as there is no averagepore diameter larger than 1000 microns. The open and total porositiesyield the effective porosity (δ_(eff)) of material, or the total amountof interconnected pore or void volume that can be filled with phasechange material, or the effective porosity (Equation 3).ε_(eff)=δ_(total)·ε_(open)  (3)The total and effective porosities of the host material can be used todetermine the effective thermal conductivity (k_(eff)) of the hostmaterial filled with phase change material (Equation 4). The effectivethermal conductivity is a function ofk_(eff)=(1−ε_(total))·k_(HM)+(ε_(total)·Ε_(open))·k_(PCM)+ε_(total)·(1−ε_(open))·k_(gas)  (4)the thermal conductivities of the host material (k_(HM)), the phasechange material (k_(PCM)) and the thermal conductivity of any gastrapped in closed pores (k_(gas)). The impact of total porosity on theeffective thermal conductivity is illustrated in FIG. 2. This figureassumes that paraffin is used as the phase change material and has athermal conductivity of k_(PCM)≅0.2 W/m-K, and that the host materialhas a thermal conductivity of k_(HM)≅540 W/m-K, and an open porosity ofε_(open)≅1. If there were no voids in the material, meaning there was nototal porosity, the effective thermal conductivity is equal to that ofsolid graphite, and as the total porosity approaches 100% the effectivethermal conductivity of the matrix would approach that of the paraffin.

The thermal conductivity (k) of the host material is preferably selectedso that the thermal resistance between the primary coolant and the phasechange material is low and heat is transferred quickly without inducinglarge temperature gradients along the heat transfer path. The operatingtemperature of most semi-conductor and electronic devices preferablydoes not increase by more than 5-20° C. during operation and ismaintained in a proper range due to the heat transfer between the heatabsorber and the device being cooled. Preferably, the thermalconductivity of the host material is about 2-3 orders of magnitudehigher than the thermal conductivity of the phase change material. In aspecific embodiment, thermal conductivity for the host material isbetween about 50 W/m-K and about 500 W/m-K. Examples of host materialswith thermal conductivities in materials such as carbon graphite anddiamond materials, which can have high thermal conductivities that canrange from about 500 W/m-K to over 2000 W/m-K. The effective thermalconductivity of the host material filled with phase change materialshould be high enough to allow a sufficient thermal heat transfer rateto meet the needs of the application. In a specific embodiment of thesubject invention, the effective thermal conductivity of the hostmaterial filled with phase change material is in the range from about 10to about 2000 W/m-K; in another specific embodiment, the effectivethermal conductivity of the host material filled with phase changematerial is in the range from about 50 to about 1500 W/m-K; and inanother specific embodiment, the effective thermal conductivity of thehost material filled with phase change material is in the range fromabout 100 to about 500 W/m-K.

The total energy storage capacity (Joules/volume) of the material is afunction of the total and open porosities of the host material, thelatent heat of fusion for the phase change material (λ_(PCM)), and thedensity of the phase change material (ρ_(PCM)) and can be evaluatedusing Equation 5. The relationship between host material porosities andE_(capacity)=λ_(PCM)·ρ_(PCM)·ε_(total)·ε_(open)  (5)thermal energy storage capacity of the heat absorber matrix can bevisualized in FIG. 3. This specific example, assumes a host material isfilled with a phase change material with a latent heat of fusion of 200kJ/kg and a liquid density of 800 kg/m³. The thermal energy storagecapacity (J/cm³) is presented for total porosities varying from 0% to100% at four specific open porosities (65%, 75%, 85%, 95%) representedby four separate curves. It is evident from this figure that highertotal porosity materials can store more thermal energy per unit volume,for the same open porosities. The latent heat of fusion, for the phasechange material used in the subject invention, λ_(PCM), is preferably inthe range from about 10 to about 350 kJ/kg, more preferably in the rangefrom about 100 to about 300 kJ/kg, and even more preferably in the rangefrom about 150 to about 250 kJ/kg. In order to maximize the energystorage capacity of the subject invention the latent heat of fusion ofthe phase change material should be as high as possible.

Several phase change materials offer latent heat characteristics thatwould be desirable for use in a heat absorber application. A specificembodiment of the subject invention can utilize organic paraffin, wherethe latent heat of fusion for organic paraffin's can range from about100 kJ/kg to about 250 kJ/kg. Inorganic salts and salt hydrate phasechange solutions can also be utilized with the subject invention andhave latent heat of fusion properties ranging from about 50 kJ/kg toabout 350 kJ/kg. Water has a latent heat of fusion value of ˜330 kJ/kgand can also be utilized with the subject invention. Consideration mustbe taken when selecting the phase change material for optimum heattransfer characteristics as well as mechanical considerations such asthe volume differences between the liquid and solid states of the phasechange material.

The effective thermal conductivity and energy storage capacity of thehost material must be optimized to meet the operating requirements ofthe device being cooled. The total porosity has the largest impact onboth the effective thermal conductivity and the thermal energy storagecapacity. In a host material that has a high porosity there is morephase change material than host material, therefore the effectivethermal conductivity will be lower, tending to result in a largertemperature gradient between the heat absorber and the electronic devicebeing cooled. If the total porosity of the host material is decreasedthe temperature gradient will also tend to be reduced, but the thermalenergy storage capacity will tend to decrease leading to a lower heatabsorption rate for a specified volume of material.

The phase change material can be placed into the host material through avariety of processes. In a specific embodiment, the phase changematerial is heated remotely until it is in a liquid form and then thehost material is immersed in the liquid phase change material until thedesired amount of phase change material is soaked into the porous hostmaterial. In another specific embodiment, dispersing the phase changematerial, involves pulling a vacuum on the host material, heating thephase change material until it reaches a liquid state and then releasingthe liquid phase change material into the host material via apressure-driven flow. Pulling a vacuum on the host material can ensuremost, if not all, of the air is removed from the void volumes in thehost material, such that a high amount of phase change material fillsthe void volumes in the host material.

A specific embodiment of the invention uses high thermal conductivitygraphite foam developed by Oak Ridge National Laboratory (Poco Graphite,Inc. manufactures two graphite foam products in accordance with thistechnology, e.g., POCO FOAM™ and POCO HTC™). U.S. Pat. No. 6,033,506teaches methods of making carbon foam that can be used as a hostmaterial in accordance with the subject invention, and is herebyincorporated by reference in its entirety. The graphite foam can beconsidered to augment the low thermal conductivity of paraffin phasechange material. The graphite foam's porous structure can allow paraffinto fill in the voids in the foam and increase the surface to volumeratio, allowing the energy to be stored and released from the PCMquickly. Poco Graphite, Inc.'s POCO FOAM™ material has an out-of-planethermal conductivity of k=145 W/m-K, a total porosity of ε_(total)=0.75,an open porosity of ε_(open) =0.96, and an average pore size of 350microns. Poco Graphite, Inc.'s POCO HTC™ graphite foam has a thermalconductivity of k=245 W/m-K in the out-of-plane direction, a totalporosity of ε_(total)=0.61, an open porosity of ε_(open)=0.95, and anaverage pore size of 350 microns. Another specific embodiment of thesubject invention uses a low temperature paraffin ASTOR™ Astorphase 3,which is manufactured by Honeywell Specialty Wax and Additives, as aplane change material. Astorphase 3 has unique material properties thatare conducive for the phase change material for a heat absorber that isused in a solid-state laser cooling system due to is low melting pointof approximately 5° C. and the relatively high latent heat of fusion of171.2 kJ/kg.

Rini Technologies, Inc. has performed successful thermal energy storagetests with Poco Graphite, Inc.'s POCO FOAM™ graphite foam filled withHoneywell's Astorphase 42 X, which has a melting point of approximately40° C. and a latent heat of fusion above 200 kJ/kg. These experimentshave shown that porous graphite foam filled with paraffin makes an idealheat absorber. During these experiments, referring to FIG. 4,thermocouples were embedded at increasing distances from the heatedsurface to record the temperature during the experiment. Theexperimental data, shown in FIG. 5, shows the temperature profile ofthree thermocouples embedded in the graphite foam, where —● representsthe first thermocouple, × represents the second thermocouple, and—▪represents the third thermocouple. In the first few seconds of theexperiment leading up to point I, a sensible heat transfer process istaking place in the solid paraffin. The phase change of the paraffintakes place between points I and II when the paraffin reaches itsmelting point and absorbs heat through a latent heat transfer process,while maintaining the temperature within the foam within a relativelysmall range. After point II, all the paraffin has melted and thetemperature begins to increase again as the liquid wax begins absorbingheat through a sensible heat transfer process. The experimental resultsclearly show that the high thermal conductivity of the graphite foamleads to a bulk melting process and a melting front which travels fromthe device surface in thermal contact with the heat source to the firstthermocouple and then to the second thermocouple and then to the thirdthermocouple is not present. In a normal phase change process, there isan interface that exists between liquid and solid regions, which movesthrough the bulk phase change material during the phase change process.Although smaller scale melting fronts are likely present, theexperimental results show that at applied heat fluxes of 5 W/cm² or lessthere is no bulk melting front moving from the end of the device inthermal contact with the heat source to the other end with the TESgeometry tested. Furthermore, all thermocouples have a relatively flatslope in the phase change region (between points I and II), indicatingthat all the paraffin is melting at approximately the same time.However, it is important to note that for thicker TES geometries orapplied heat fluxes greater than 5 W/cm² a bulk melting front wouldlikely be present.

The subject invention pertains to a method and apparatus for storingthermal energy. The subject invention also relates to a method andapparatus for absorbing thermal energy from a heat source. In a specificembodiment, the subject thermal energy storage apparatus can function asa heat absorber in a cooling system. The subject heat absorber canabsorb large amounts of waste heat over short time periods, such as timeperiods of less than about a minute, and then slowly reject that heat tothe ambient environment over longer time periods, such as time periodsover about 30 minutes. FIGS. 6A and 6B schematically illustrate a heatabsorber 1 in accordance with the subject invention. The heat absorber1, shown in FIGS. 6A and 6B, includes a phase change material, which isstored in the pores 2 of the host material. The host material 3 thatencloses the phase change material 2 can be, but is not limited to, ametal foam or graphite foam material that has a high thermalconductivity, k. In a specific embodiment, the host material filled withphase change material has an effective thermal conductivity of at leastabout 100 W/m-K. The subject heat absorber can be manufactured in avariety of geometric shapes to accommodate the arrangement of thecooling system.

A specific configuration of a heat absorber in accordance with thesubject invention is shown in FIGS. 7A and 7B. A heat source 4, such asa heated electronic device, a laser diode, a computer processing chip,or other heat generating electronic device, is in thermal contact withthe heat absorber 1. Thermal contact can be achieved by, for example,directly mounting the heat source to the heat absorber. The heatgenerated by the electronic device is conductively transferred to theheat absorber surface. The high thermal conductivity host material 3quickly disperses the heat throughout the heat absorber. If enough heatis absorbed, the heat melts the phase change material 2 that is enclosedin the pores. The heat absorber volume should be appropriately sized forthe operation of the electronic device, ensuring an adequate amount ofphase change material is present to melt and store the waste heat.Following the nominal operation of the electronic device the heat isrejected from the heat absorber by some method of, for example,conduction, convection, and/or radiation. This heat can be rejected froma separate surface on the absorber similar to that shown in FRONT VIEWof FIG. 7A. In a specific embodiment, a cold plate can be mounted to thebottom surface. The cold plate can slowly solidify the melted phasechange material effectively removing the stored energy from the heatabsorber.

Another specific embodiment of the subject invention can incorporate thesubject heat absorber into a closed loop cooling cycle that has twoseparate cooling loops, such as a primary cooling loop 16 and asecondary cooling loop 18. The primary loop and/or the secondary loopcan be any of many means known in the art for removing heat. Examplesinclude a water chiller, a refrigeration cycle, a cold liquid pump pool,and spray-cooling evaporator. Although FIG. 8 shows primary coolanttubing 6 and secondary coolant tubing 8 entering and leaving heatabsorber 7 at different locations, alternative embodiments can utilizeone set of coolant tubing traveling through the heat absorber 7. Forexample, the primary coolant can flow through the set of coolant tubingand then, via valving and/or switching known in the art, the primarycoolant can be turned off and the secondary coolant can be allowed toflow through the coolant tubing. In this way the primary and secondarycoolants can share coolant tubing.

Referring to FIG. 8 in a closed cycle configuration the primary coolingloop can acquire the waste heat generated by the electronic device 4, orother heat source, through a heat exchanger device functioning as anevaporator 5. In a specific embodiment, the heat exchanger 5 can be asingle-phase sensible heat exchanger where coolant flows through theheat exchanger increasing the primary coolant temperature whileabsorbing thermal energy from the heat source. In another specificembodiment, the heat exchanger can be an evaporative heat exchanger 5that utilizes a two-phase cooling process that takes advantage of thelatent heat of vaporization to dissipate the heat load through the phasechange of the primary coolant. Specific examples of these two types ofevaporators can be, but are not limited to, water liquid heatexchangers, micro-channel coolers, evaporative spray cooling nozzlearrays, or small heat absorbing units. Evaporative spray coolingtechniques can involve spraying via one or more spray nozzles a coolantonto a surface such that the coolant absorbs heat and flows and/orevaporates from the surface so as to remove heat from the surface. Theprimary coolant can use traditional cooling fluids such as water,ammonia, refrigerants like R-134 a or R22, or other appropriate coolingfluids. The heated or vaporized primary coolant is transferred to theheat absorber 7 by plumbing or tubing 6 that can have, for example, acircular, square, rectangular, other cross-sectional shape. The plumbingor tubing 6 can be fabricated from materials such as, but is not limitedto, aluminum, stainless steel, and copper. Preferably, the material iscompatible with the working fluid in the primary cooling loop and heatcan be transferred through the plumbing or tubing 6 wall. The primarycoolant is then in thermal contact with the host material of the heatabsorber. In a specific embodiment, plumbing can be embedded in the heatabsorber host material to allow the primary and secondary cooling loopsto transfer heat to and from the heat absorber device. Techniques forembedding the coolant tubing, or plumbing, in the heat absorber can beused to enhance the thermal energy transfer from the tubing to the heatabsorber host material and/or phase change material. Such techniquesinclude, but are not limited to, using a molten material that solidifiesto bond the tubing to the material, using thermal epoxies to bond thetubing to the host material, soldering the tubing to the host material,and other techniques known in the art for creating better contactbetween the tubing and the host material.

FIGS. 9A and 9B schematically illustrate a cross-section of anembodiment of the subject heat absorber incorporating embedded plumbing,which assists the heat transfer between the host material and thecoolant. The heat from the primary coolant loop enters the heatabsorbers through a series of primary coolant channels 13 that areembedded in the thermal heat absorber 1 as shown in FIGS. 9A and 9B. Theprimary coolant traveling in the primary coolant channels 13 is cooledor condensed removing heat from the primary cooling loop and effectivelystoring the heat in the heat absorber 7 unit allowing the primarycoolant to return to the evaporator 5 to continue to cool the electronicdevice 4.

Referring again to FIGS. 9A and 9B, a specific embodiment of a closedcycle heat absorber 7 configuration is depicted. An external frame orhousing 12 can be incorporated to prevent the phase change material 2from leaking out of the porous heat absorber heat material 3. The frame12 can be fabricated from a variety of materials such as, but notlimited to metals, plastics, and composite materials. Preferably, theframe material is compatible with the host material and the phase changematerial and provides adequate structural support. In the embodimentshown in FIGS. 7A and 7B, the heat from the primary coolant loop istransferred through the wall of the primary coolant channels 13 to thehigh thermal conductivity host material 3, which disperses the heatquickly throughout the volume of the heat absorber 7. In a specificembodiment, the heat absorber is designed such that the heat isdispersed throughout the volume of the heat absorber within about aminute, melting the phase change material 2 and absorbing the heat fromthe primary coolant through a latent heat transfer process.

Referring again to FIGS. 9A and 9B, the energy stored in the closedcycle heat absorber 7 is slowly removed by a secondary coolant loop 18through a heat rejector 9 over an extended period of time. A specificembodiment of the heat rejector 9 would be a cold temperature liquidpump loop or in another embodiment a standard vapor compressionrefrigeration cycle, or some other cooling method that removes the heatfrom the secondary cooling loop 18. In a specific embodiment, the heatremoval period is at least about 10 times longer than the absorptionperiod. In additional embodiments, the heat removal period is at leastabout 2, 5, 10, 20, 30, 50, and 100 times as long as the absorptionperiod, respectively. A standard refrigerant such as R-22, R-134 a, orammonia can be used as the secondary coolant. Heat is transferred fromthe phase change material through the walls of the second set ofembedded plumbing channels 14 that freeze or solidify the phase changematerial 2 transferring the heat to the secondary cooling channels 14through a latent heat of vaporization process. Referring to FIG. 8, thevaporized secondary coolant is transferred by plumbing or tubing 8 tothe heat rejector 9, which removes the heat stored in the secondarycooling loop 18 allowing the secondary coolant to condense and bereturned to the heat absorber 7.

As is apparent to one skilled in the art, the subject heat absorber canbe incorporated with a variety of types of cooling systems in a varietyof configurations.

All patents, patent applications, provisional applications, andpublications referred to or cited herein are incorporated by referencein their entirety, including all figures and tables, to the extent theyare not inconsistent with the explicit teachings of this specification.

It should be understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication.

1. A method for absorbing thermal energy from a heat source, comprising:absorbing thermal energy from a heat source via a heat exchanger so asto transfer thermal energy from the heat source to a primary coolant;transporting the primary coolant from the heat exchanger to a heatabsorber so as to transfer thermal energy from the primary coolant tothe heat absorber, wherein transporting the primary coolant from theheat exchanger to a heat absorber comprises transporting the primarycoolant from the heat exchanger to a heat absorber comprising a phasechange material, wherein transfer of thermal energy from the primarycoolant to the heat absorber melts at least a portion of the phasechange material, wherein the heat absorber comprises a host materialhaving voids that are filled with the phase change material such thatthe phase change material is in thermal contact with the host material,wherein the heat absorber comprises primary coolant tubing embedded inthe heat absorber, wherein the primary coolant tubing is in thermalcontact with at least a portion of the host material, wherein theprimary coolant travels through the primary coolant tubing, whereinthermal energy is transferred from the primary coolant to the phasechange material through the primary coolant tubing; and removing thermalenergy from the heat absorber, wherein removing thermal energy from theheat absorber comprises: transferring thermal energy from the heatabsorber to a secondary coolant, wherein the heat absorber comprisessecondary coolant tubing embedded in the heat absorber, wherein thesecondary coolant tubing is in thermal contact with at least a secondportion of the host material, wherein the secondary coolant travelsthrough the secondary coolant tubing, wherein thermal energy istransferred from the phase change material to the secondary coolantthrough the secondary coolant tubing.
 2. The method according to claim1, wherein absorbing thermal energy from a heat source via the heatexchanger comprises absorbing thermal energy from a heat source at afirst rate of thermal energy transfer during a first period of time,wherein dissipating thermal energy from the heat absorber comprisesdissipating thermal energy from the heat absorber at a second rate ofthermal energy transfer during a second period of time, wherein thefirst rate of thermal energy transfer is higher than the second rate ofthermal energy transfer, and the first period of time is shorter thanthe second period of time.
 3. The method according to claim 2, whereinthe second period of time is at least 2 times as long as the firstperiod of time.
 4. The method according to claim 2, wherein the secondperiod of time is at least 5 times as long as the first period of time.5. The method according to claim 2, wherein the second period of time isat least 10 times as long as the first period of time.
 6. The methodaccording to claim 2, wherein the second period of time is at least 20times as long as the first period of time.
 7. The method according toclaim 2, wherein the second period of time is at least 50 times as longas the first period of time.
 8. The method according to claim 2, whereinthe second period of time is at least 100 times as long as the firstperiod of time.
 9. The method according to claim 1, wherein the heatsource comprises a laser.
 10. The method according to claim 1, whereinthe phase material comprises a paraffin.
 11. The method according toclaim 1, wherein the host material has a total porosity in the rangefrom about 0.6 to about 0.75.
 12. The method according to claim 1,wherein the host material has an open porosity in the range from about0.8 to about 1.0
 13. The method according to claim 1, wherein the hostmaterial has an average pore size in the range from about 300 microns toabout 400 microns.
 14. The method according to claim 1, wherein the hostmaterial filled with the phase change material has an effective thermalconductivity in the range from about 100 W/m-K to about 500 W/m-k. 15.The method according to claim 1, wherein the host material filled withphase change material has an effective thermal conductivity of at least100 W/m-K.
 16. The method according to claim 1, wherein the heatexchanger is an evaporative heat exchanger, wherein thermal energytransferred to the primary coolant from the heat source vaporizes atleast a portion of the primary coolant.
 17. The method according toclaim 16, wherein the evaporative heat exchanger is a spray coolant heatexchanger.
 18. The method according to claim 1, wherein the secondarycoolant tubing comprises the primary coolant tubing.
 19. The methodaccording to claim 1, wherein the primary coolant tubing has a circularcross-sectional shape.
 20. The method according to claim 1, wherein theprimary coolant tubing has a rectangular cross-sectional shape.
 21. Themethod according to claim 1, further comprising removing heat from thesecondary coolant.
 22. An apparatus for absorbing thermal energy from aheat source, comprising: a primary coolant; a heat exchanger, whereinthermal energy is absorbed from the heat source and transferred to theprimary coolant via the heat exchanger; a heat absorber, wherein theprimary coolant is transported from the heat exchanger to the heatabsorber, wherein thermal energy is transferred from the primary coolantto the heat absorber, wherein the heat absorber comprises a phase changematerial, wherein transfer of thermal energy from the primary coolant tothe heat absorber melts at least a portion of the phase change material,wherein the heat absorber comprises a host material having voids thatare filled with the phase change material such that the phase changematerial is in thermal contact with the host material, wherein the heatabsorber comprises primary coolant tubing embedded in the heat absorber,wherein the primary coolant tubing is in thermal contact with at least aportion of the host material, wherein the primary coolant travelsthrough the primary coolant tubing, wherein thermal energy istransferred from the primary coolant to the phase change materialthrough the primary coolant tubing; and a secondary coolant, whereinthermal energy from the heat absorber is transferred to the secondarycoolant, wherein the heat absorber comprises secondary coolant tubingembedded in the heat absorber, wherein the secondary coolant tubing isin thermal contact with at least a second portion of the host material,wherein the secondary coolant travels through the secondary coolanttubing, wherein thermal energy is transferred from the phase changematerial to the secondary coolant through the secondary coolant tubing.23. The apparatus according to claim 22, wherein the evaporative heatexchanger is a spray coolant heat exchanger.
 24. The apparatus accordingto claim 22, wherein thermal energy is absorbed from the heat source ata first rate of thermal energy transfer during a first period of time,wherein thermal energy is transferred from the heat absorber to thesecondary coolant at a second rate of thermal energy transfer during asecond period of time, wherein the first rate of thermal energy transferis higher than the second rate of thermal energy transfer, and the firstperiod of time is shorter than the second period of time.
 25. Theapparatus according to claim 24, wherein the second period of time is atleast 2 times as long as the first period of time.
 26. The methodaccording to claim 24, wherein the second period of time is at least 5times as long as the first period of time.
 27. The method according toclaim 24, wherein the second period of time is at least 10 times as longas the first period of time.
 28. The method according to claim 24,wherein the second period of time is at least 20 times as long as thefirst period of time.
 29. The method according to claim 24, wherein thesecond period of time is at least 50 times as long as the first periodof time.
 30. The method according to claim 24, wherein the second periodof time is at least 100 times as long as the first period of time. 31.The apparatus according to claim 22, wherein the heat exchanger is anevaporative heat exchanger, wherein the thermal energy transferred tothe primary coolant from the heat source vaporizes at least a portion ofthe primary coolant.
 32. The apparatus according to claim 22, whereinthe phase change material comprises a paraffin.
 33. The apparatusaccording to claim 22, wherein the host material has a total porosity inthe range from about 0.6 to about 0.75.
 34. The apparatus according toclaim 22, wherein the host material has an open porosity in the rangefrom about 0.8 to about 1.0.
 35. The apparatus according to claim 22,wherein the host material has an average pore size in the range fromabout 300 microns to about 400 microns.
 36. The apparatus according toclaim 22, wherein the host material filled with the phase changematerial has an effective thermal conductivity in the range from about100 W/m-K to about 500 W/m-k.
 37. The apparatus according to claim 22,wherein the secondary coolant tubing comprises the primary coolanttubing.
 38. The apparatus according to claim 22, wherein the primarycoolant tubing has a circular cross-sectional shape.
 39. The apparatusaccording to claim 22, wherein the primary coolant tubing has arectangular cross-sectional shape.
 40. The apparatus according to claim22, wherein the heat exchanger is an evaporative heat exchanger, whereinthermal energy transferred to the primary coolant from the heat sourcevaporizes at least a portion of the primary coolant.